Optical networks employing wavelength division multiplexing (WDM) are now receiving a great deal of attention due to their ability to carry enormous amounts of information over a single optical fiber. Such networks typically require the monitoring and adjustment of the power levels of each wavelength component in order to produce a balanced performance. This automatic power control is usually performed by discrete mechanical attenuators in the optical cross-connects or in other network nodes where the signals are demultiplexed into separate waveguides.
The mechanically controlled attenuators tend to be large and expensive. In addition, they are usually too slow to remove (compensate) rapid fluctuations in the optical power which result, for example, from polarization-dependent insertion loss in the optical components of the transmission line (including the optical switch) and random variations in the state of polarization of the optical signals. Such fluctuations might also occur in optically amplified WDM transmission systems when one or several wavelength channels are added or dropped.